Most occupied buildings require venting. The type of venting employed depends on the kind of enclosure to be vented. For example, bathrooms containing showers typically have active vents with fans to vent steam outside. Other types of enclosures, such as attics, include a passive vent to allow for air flow from the enclosure to the atmosphere, to prevent moisture build-up. It is advantageous if the vent allows for the flow of air through the passage, without permitting moisture, such as rain or snow, to enter the enclosure through the passage. Structures are known that achieve this goal, but they can be relatively costly.